The need exists in a large number of fields to perform grinding operations on workpieces. For example, a billet is often the raw material for a rolling process in which the billet is flattened and formed into a finished product. These billets often contain surface imperfections which, if not removed, are carried through to the finished product. Accordingly, these imperfections are normally removed in a grinding process called "spotting" in which a rotating grinding wheel is held against the surface imperfection until the surface imperfection is removed. The surfaces of billets are also normally coated with a layer of oxides and other material. This surface layer must also be removed in order to prevent the surface layer from degrading the quality of the finished product. The surface layer is normally removed in a process called "skinning" in which the billet reciprocates beneath the grinding wheel while the grinding wheel is held in contact with the billet. The skinning process is often performed automatically. The grinding pressure of the wheel against the workpiece is automatically controlled while the workpiece reciprocates back and forth adjacent the grinding wheel, and the grinding wheel steps across the workpiece an incremental distance each grinding pass.
The automatic skinning process is generally performed only on the top surface of the workpiece. In order to grind all four surfaces of a rectangular workpiece, the workpiece is sequentially rotated from one surface to the next so that the surface to be skinned is facing upwardly.
Grinding the upper surface of a workpiece is relatively straightforward since the grinding force exerted by the grinding wheel in a downward direction can be adjusted by a control signal derived from such parameters as a horsepower or grinding force. It is possible to use such techniques even though the workpiece is curved or bowed upwardly since the grinding wheel is automatically moved in a vertical direction to maintain the horsepower, grinding force, or other parameter at a target value. In contrast, grinding the sides of a workpiece presents problems not encountered with grinding the top surface of the workpiece. Although operating parameters such as grinding power or grinding force can be used to control actuators moving the grinding wheel in a horizontal direction, bowing of the workpiece in a vertical direction prevents the grinding wheel from uniformly contacting the sides of the workpiece. In other words, for a workpiece that is bowed upwardly, a grinding wheel having a constant vertical position would contact the upper portion of the workpiece side at the ends of the workpiece, but it might contact the lower portion of the side at the center of the workpiece. Although workpiece grinders having these characteristics are sometimes used, the nonuniformity in the manner that the grinding wheel contacts the workpiece degrades the quality of the automatic skinning process.
Although conventional grinding operations are being described in some detail with respect to billets, it will be understood that the principles of the invention applicable to billets are also equally applicable to other grinding processes.